The present invention relates to an automatic gain and frequency characteristic control unit in an audio device which automatically controls the amplification gain and frequency characteristic according to the level of external noise.
In an automobile, sound such as music outputted by an audio device is often masked by the ambient noise. In order to prevent such masking, an automatic gain control unit is available which can automatically control the amplification gain according to the level of the noise.
A conventional automatic gain control unit, as shown in FIG. 1, includes a noise detecting section 1-1 and a gain varying circuit 1-2. In the noise detecting section 1-1, external noise is picked up by a microphone 1-3, and the output signal of the microphone 1-3 is supplied through a microphone amplifier 1-4 to an LPF (low-pass filter) 1-5 which extracts only the noise band component. The output signal of the LPF 1-5 is rectified by a diode 1-6 and smoothed by a smoothing circuit 1-7, the output of which is supplied to a logarithmic compression circuit 1-8. The output signal of the logarithmic compression circuit 1-8 is supplied, as a gain control signal from the noise detecting section 1-1, to the gain varying circuit 1-2. The gain varying circuit 1-2 is implemented with an electronic volume control circuit, for instance. The gain varying circuit 1-2 is inserted in a signal line which transmits an audio signal from a tape deck or tuner to a power amplifier to thereby vary the amplification gain of the audio signal according to the output signal level of the logarithmic compression circuit 1-8. Accordingly, as the external noise level increases, the amplification gain of the audio signal is increased, and the acoustic level of the sound reproduced by the audio device is also increased.
In the case where the conventional automatic gain control unit is applied to an audio device mounted on a vehicle, as the noise level increases linearly (as indicated by the solid line a in FIG. 2) with the travel speed of the vehicle (noise level), the average volume level of a sound reproduced by the audio device is increased linearly (as indicated by the solid line b), thereby to prevent the masking phenomenon. In FIG. 2, the broken lines indicate the range in which the volume level varies.
In the conventional automatic gain control unit, gain control is carried out according to the noise level only, that is, it is performed independently of the audio signal level or dynamic range. In the case where the audio signal has a narrow dynamic range, the masking correction effect is sufficiently high. However, for music such as a symphony which has a wide dynamic range, the reproduced sound of the low level part of the audio signal is masked by noise as indicated by an arrow c in FIG. 2. If the gain is increased in such a manner that the reproduced sound of the lower level part of the audio signal is not masked by the noise, then the high level part of the audio signal becomes excessively large in volume.
In order to overcome this difficulty, an automatic gain control unit has been proposed which, as shown in FIG. 3, includes a noise detecting section 1-1, a gain varying circuit 1-2, a signal detecting section 1-9 and an arithmetic circuit 1-10. The noise detecting section 1-1 and the gain varying circuit 1-2 are the same as those in FIG. 1. In the signal detecting section 1-9, an audio signal supplied to the gain varying circuit 1-2 is rectified by a diode 1-11 and smoothed by a smoothing circuit 1-12, and the rectified voltage is subjected to a logarithmic compression by a logarithmic compression circuit 1-13 to provide a signal detection output. The arithmetic circuit 1-10 is composed of a multiplier 1-14 with which the output voltage of the signal detecting section 1-9 is multiplied by a voltage V.alpha. corresponding to a negative constant .alpha., an adder 1-15 for adding a voltage V.beta. corresponding to a positive constant .beta. to the output voltage of the multiplier 1-14, and a multiplier for obtaining the product of the output voltage of the adder 1-15 and the output voltage of the noise detecting section 1-1. The output of the multiplier 1-16 is supplied as a gain control signal to the gain varying circuit 1-12. The constant determines the increment in compression degree of an audio signal level with respect to an increment of a noise level; for instance -0.04&lt;.alpha.&lt;0. The constant .beta. determines an increment in gain of an audio signal reference level with respect to an increment of a noise level; for instance, 0.ltoreq..beta..ltoreq.1.
If it is assumed that, in the conventional automatic gain control unit thus constructed, a noise level increment with respect to a reference noise level, which is provided by the noise detecting section 1-1, is represented by .DELTA.N, and an audio signal level increment with respect to a reference signal level, which is provided by the signal detecting section, is represented by .DELTA.S, then the output voltage of the arithmetic circuit 1-10 is proportional to .DELTA.N(.beta.+.alpha..DELTA.S), and the gain .DELTA.G(dB) of the audio signal by the gain varying circuit 1-2 is controlled so as to satisfy .DELTA.G=.DELTA.N(.beta.+.alpha..DELTA.S). That is, automatic volume control is carried out in such a manner that, as the audio signal level decreases, the gain is increased, and, as the audio signal level increases, the gain is decreased.
In the case where the above-described automatic gain control unit is applied to an audio device mounted on a vehicle, as the travel noise increases (as indicated by the solid line d in FIG. 4), similarly to the case of FIG. 2, the average volume level (the solid line e) of sound reproduced by the audio device is also increased with the noise level. The volume level variation range is decreased as the noise level increases, as indicated by the broken lines. This action prevents the difficulties that the low level part of the audio signal is masked by noise and the high level part is excessively high in volume. Thus, the reproduced sound is suitable in volume.
However, the conventional automatic gain control unit suffers from the problem that the reproduced sound may not always be satisfactory in hearing sensation.
Further, the invention relates to amplifier devices, and more particularly to an amplifier device in which the gain and the frequency characteristic can be changed simultaneously.
A conventional device of this type, as shown in FIG. 5, includes an electronic volume control circuit 2-1 which can vary a gain according to a control signal I, and a electronic tone control 2-2 which can increase or decrease the bass or treble frequencies in response to a control signal II. In the conventional amplifier device, when a signal is applied to an input terminal 2-3, the gain can be changed by the electronic volume control 2-1, and then the bass or treble frequencies can be adjusted by the electronic tone control 2-2.
The conventional amplifier device is disadvantageous in that the circuit is intricate because the device requires the gain controlling electronic volume and the bass and treble adjusting electronic tone control, and because the device must be provided with two control signal systems.
Still further, the invention relates to an automatic sound volume and frequency characteristic adjusting device which adjusts the sound volume and frequency characteristic of an audio device automatically according to the level of ambient noise around the audio device. More particularly, the invention relates to a noise correcting device which, according to the noise level in a vehicle, adjusts the sound volume and the sound frequency characteristic.